1. Field of the Invention
The present invention relates to a process for producing polyphosphates, such as a pyrophosphate, a tripolyphosphate, a hexametaphosphate, and the like, of an alkali metal. Alkali metal polyphosphates are useful as builders for detergents or as food additives.
2. Discussion of the Background
Alkali metal polyphosphates have conventionally been produced by starting with a solution of high purity phosphoric acid obtained by, for example, the wet process. Known processes for the production of an alkali metal polyphosphate include a two-stage process consisting of spray drying and calcination and a one-stage process consisting of spray combustion. Modifications have been introduced to obtain crystals with improved solubility. As for production of a polyphosphate starting with a phosphoric acid solution containing organic matter, a process comprising adding an oxidizing agent to phosphoric acid manufactured by the wet process which contains up to 3% by weight of organic matter based on P.sub.2 O.sub.5, followed by heating at 300.degree. to 500.degree. C. (see JP-B-56-49847, the term "JP-B" as used herein means an "examined Japanese patent publication"). According to this process, the permissible content of organic matter of the starting phosphoric acid is limited. If the organic content is too high, the organic matter cannot be eliminated sufficiently. If the phosphoric acid to be heated at the above-mentioned temperature has an organic content exceeding the limit, an extra step for removing residual organic matter, such as crystallization or the like, would be required and, in addition, the organic matter thus separated should be given such a treatment as an activated sludge process. This will complicate the whole process and increase the cost.
Generally, alkali metal polyphosphates having various degrees of phosphoric acid condensation are produced by controlling the molar ratio of an alkali metal element to an orthophosphate group (PO.sub.4) (hereinafter referred to as an M/P molar ratio). For example, acid pyrophosphates are produced at an M/P molar ratio of 1.0 and at a heating temperature of 200.degree. to 250.degree. C.; hexametaphosphates at an M/P molar ratio of 1.0 and at a temperature of 300.degree. to 500.degree. C.; tripolyphosphates at an M/P molar ratio of 1.7 at a temperature of 300.degree. to 550.degree. C.; and pyrophosphates at an M/P molar ratio of 2.0 and at a temperature of 300.degree. to 550.degree. C. (see Ullmann's Encyclopedia of Industrial Chemistry, Vol. A19, pp. 87-492).
On the other hand, a number of methods concerning combustion of a solution containing organic matter have been proposed to date. For example, a method comprising burning waste water containing organic matter and inorganic matter to make it harmless and recovering valuable inorganic matter is known (see JP-B-55-10803). Recovery of valuable inorganic matter from fermentation waste from an alcohol fermentation system, a glutamic acid fermentation system, an organic acid fermentation system, or the like, has also been practiced (JP-B-55-11848) However, the ideas of these techniques are confined to recovery of inorganic matter in its original form, not referring to recovery of an alkali metal polyphosphate from a phosphoric acid solution containing organic matter while eliminating unnecessary organic matter.
With the recent increase in concern about the environment, the regulation on phosphorus in waste water has been made more and more stringent. In industrial sectors using phosphorus, the importance of effectively utilizing phosphorus in waste water has been increasing significantly. It is important for environmental conservation to eliminate organic matter from a solution containing phosphoric acid and organic matter and to produce and collect an industrially useful polyphosphate therefrom. It is also important to produce various polyphosphates in the same equipment by controlling the degree of condensation, namely the number of phosphorus atoms per molecule, and to collect the polyphosphates in the form meeting the end use desired (for example, a solution, a slurry, crystals, or the like).